1. Field of the Invention
The present invention relates to an electrostatic deflection type cathode ray tube which has vertical and horizontal deflection electrodes which are formed in various patterns and applied to the inner surfaces of a glass bulb.
2. Description of the Prior Art
FIG. 4 is an example of a typical image pickup tube of the prior art of the magnetic focus/electrostatic deflection type and illustrates a glass bulb 1 which has a face plate 2 and a target surface 3 (photoelectric conversion surface) and an indium seal 4 for cold sealing and a metal ring 5. A signal removing metal electrode 6 passes through the face plate 2 and makes electrical contact with the target surface 3.
A cathode K is mounted in the bulb and a first grid electrode G1 and a second grid electrode G2 form an electron gun. A beam limiting aperture LA is formed in the grid G2 so as to limit the angle of divergence of the electron beam B.sub.m.
A third electrode G3 forms a deflection electrode and the electrode G3 is made with a process wherein metal such as chromium or the like is evaporated or plated on the inner surface of the glass bulb 1 and then prescribed patterns are formed by means of cutting using a laser beam or other cutting means so as to form vertical deflection electrodes V+ and V- and horizontal deflection electrodes H+ and H-. The vertical and horizontal deflection electrodes are formed in a so-called leaf pattern as illustrated in FIG. 5 or in so-called arrow patterns as illustrated in FIG. 6.
In FIGS. 5 and 6, gaps between the electrodes without metal coating are illustrated by black lines for simplification of the drawings. In FIG. 5, the hatched portions are unrequired portions to which, for example, the center voltage of the deflection voltage is applied. In FIG. 6, the hatched portions are surplus portions and are averaged in the axial direction between the electrodes H+ and H- and between the electrodes V+ and V-. Since the electrodes V+, V-, H+ and H- are formed in the patterns illustrated, the areas become cosine distributions with respect to the circumferential direction and thereby uniform deflection fields are obtained.
In FIG. 4 a mesh electrode G4 is supported by a mesh holder 7 and a focusing coil 8 is mounted external of the bulb 1 and a stem pin 9 extends through the bulb 1.
In the prior art image pickup tube with the patterns illustrated in FIGS. 5 and 6 for the electrode G3 which shows the individual electrodes V+, V- and H+ and H- which have the same area and shape, the deflection sensitivity is equalized in the vertical direction and the horizontal direction.
In an actual tube, the deflection scanning of the electron beam B.sub.m the aspect ratio is not 1:1 but is 4:3 or 5:3. Thus, if the deflection sensitivity is made to be equal in the vertical and horizontal directions as in the prior art tubes, a circuit to drive the deflection electrodes requires that the deflection source voltage be sufficient to drive the larger deflection direction and elements must be provided which can withstand the required voltage. For example, when scanning with an aspect ratio of 5:3, if the vertical deflection voltage is 100 volts (peak-to-peak) the horizontal deflection voltage must be 167 V (peak-to-peak) and the deflection source voltage must be 167 V+2. For this case, a surplus voltage is produced for the vertical deflection and it is not preferable from the viewpoint of maintaining low power consumption.
If the deflection sensitivity is selected to be 1.3 times in the horizontal direction and 1/1.3 times in the vertical direction without varying the overall length of the deflection electrode G3, the deflection voltage will be about 130 volts (peak-to-peak) in the horizontal direction and the vertical direction which thereby decreases the source voltage and results in lower power consumption and the breakdown voltage of the circuit elements may be selected to be lower.